All products certified suitable for use in a hazardous area, whether they be instruments, enclosures, luminaries etc should be marked with their temperature class. The temperature class defines the maximum surface temperature the product is allowed to operate at. Temperature class is referenced to an ambient of -20°C to +40°C unless specifically indicated otherwise.
According to the type of explosion protection used on the product e.g. Exd, Exe etc, the temperature corresponds either to the maximum temperature of the external surface of the product, or to the maximum temperature of the internal surface of the product.
When selecting a product, ensure the temperature class of the product is below the minimum ignition temperature of the explosive atmosphere where it will be installed.
North American and Canadian practice differs from that used in Europe with smaller temperature ranges being allowed in the new world.
The European codes allow for a Small Component Relaxation as below;
|Surface Area ||T4 Classification|
|< 20mm2||Surface temperature < 275°C |
| >20mm2 <10cm2 ||Surface temperature <200°C |
| >20mm2 ||Power dissipation <1.3 W at 40°C ambient|
Power dissipation <1.2 W at 60°C ambient
Power dissipation <1.0 W at 80°C ambient